Liquefying natural gas with low pressure refrigerants



N V. 22, 1960 L. K. SWENSON ETVAL 2,960,837

LIQUEFYING NATURAL GAS WITH LOW PRESSURE REFRIGERANTS Filed July 16,1958 3 Sheets-Sheet 1 I I I Q Q? Q Q) g M q INVENTORS [donan/ KT Slu/75012 By James Debug LIQUEFYING NATURAL GAS WITH Low PRESSUREREFRIGERANTS Filed July 16, 1958 Nov. 22, 1960 L. K. SWENSON EIAL 3Sheets-Sheet 3 Jame & INVENTORS nite States Patent LIQUEFYENG NATURALGAS WlTH LOW PRESSURE REFRIGERANTS Leonard K. Swanson and James De Lory,Kansas City,

Mo., assignors, by mesne assignments, to Conch International MethaneLimited, Nassau, Bahamas, a corporation of the Bahamas Filed July 16,1958, Ser. No. 748,888

16 Claims. (Cl. 62-24) This invention relates to the liquefaction of agas and, more particularly, to a method and apparatus for theliquefaction of natural gas which is normally composed mostly of methanebut which may contain heavier hydrocarbons such as ethane, propane,butane and the like, small amounts of aromatic hydrocarbons and variableamounts of non-hydrocarbons, such as nitrogen, helium, carbon dioxide,hydrogen sulfide and the like. Illustration of this invention willhereafter be made with reference to the liquefaction of natural gas butit will be understood that the concepts employed are also capable ofapplication to other low boiling liquefiable gases, such as nitrogen,helium, air, oxygen and the like.

There are many purposes for which natural gas is desired to be reducedto a liquefied state. The main reason resides in the resultantreduction, at equivalent pressure, by about in volume when reduced fromthe gaseous state to a liquefied state, thereby to enable storage andtransportation in containers of more economical and practical design.

For example, when gas is transported by pipeline from the source ofsupply to a distant market, it is desirable to operate undersubstantially constant high load factor. Often times the flow capacitywill exceed demand, while at other times the demand may exceed thecapacity of the line. In order to shave off the peaks where demand wouldexceed supply, it is desirable to store gas when the supply exceedsdemand, whereby peaks in demand can be met by material in storage. Forthis purpose, it is desirable to provide for storage in a liquefiedstate and to vaporize liquid in amounts to meet demand.

Liquefaction of natural gas is of even greater importance in making itpossible to transport the gas from a source of plentiful supply to adistant market where a deficiency exists, especially when the source ofsupply cannot be directly joined with the market by a pipeline or thelike means for the transportation of the gaseous fuel in a gaseousstate. By way of illustration, surplus natural gas is available in theGulf States of the United States, in Venezuela, and in the Persian Gulf,while deficiencies exist in the northern parts of the United States, theEuropean countries, and Japan, yet these sources of supply cannot bejoined by pipeline with some of the markets. Ship transportation in thegaseous state would be uneconomical, unless the gaseous materials werecompressed, and then the system would not be commercial because it wouldbe impractical to provide containers of suitable strength and capacity.

It has been determined that natural gas, when shipped from the UnitedStates or Venezuela in large volumes in liquefied state, can be madeavailable in Great Britain, for example, at a price which isconsiderably less than locally manufactured gas. For shipment in largevolume, it is desirable to house the liquefied natural gas in suitableinsulated containers of large capacity at about atmospheric pressure, orpreferably slightly above atmospheric, but not at such high pressures aswould unduly limit the economical capacity of the tank. Depending uponthe amount of higher boiling heavier hydrocarbons present in the naturalgas, the liquefied natural gas will have a boiling point within therange of 240 F. to 258 F. at atmospheric pressure.

The present invention contemplates a novel method of converting a gasfrom a gaseous state to a liquefied state for storage or transportation,wherein the gas is cooled in incremental steps by use of a plurality ofrefrigerants in such a manner that each refrigerant is used to removethe maximum amount of heat from the gas. Each refrigerant is compressedand expanded in a separate cycle, with each refrigerant being passed inheat exchange relation with the gas being liquefied in a plurality ofstages at progressively decreasing pressures of the refrigerant, suchthat each refrigerant is used at the most efficient temperature levelsfor removing heat from the gas being liquefied. Each refrigerant is alsocompressed in a plurality of stages, such that a refrigerant expanded atintermediate pressure stages and passed in heat exchange relation withthe gas being liquefied may be by-passed around the lower stages ofcompression and increase the economy of the refrigeration cycle. Thisinvention further contemplates the distillation of nitrogen from anatural gas being liquefied in such a manner that heat from the naturalgas being liquefied and refrigeration from one of the refrigerants usedfor liquefying the natural gas are used to maintain the desiredtemperatures in 2. nitrogen stripping tower for the efficient removal ofnitrogen from the natural gas. In a preferred embodiment, the naturalgas is used to heat the lower section of a nitrogen stripping tower andis then further cooled by a refrigerant before being expanded into thenitrogen tower for nitrogen removal.

An important object of this invention is to provide an economical andefficient method for converting a gas, and particularly a natural gas,from a gaseous state to a liquefied state for storage andtransportation.

Another object of this invention is to provide a method of liquefying agas by use of a plurality of refrigerants, wherein each refrigerant iscirculated through a separate cycle and passed in heat exchange relationwith the gas being liquefied at temperature levels which provide themost efiicient removal of heat from the gas being liquefied.

Another object of this invention is to provide a method of liquefyingnatural gas by use of a plurality of separate refrigeration cycles,wherein each refrigeration cycle utilizes a plurality of stages ofcompression and expansion and wherein the heat of compression in thelower temperature level refrigeration cycles is at least partiallyremoved by a higher temperature level refrigerant which is also used forremoving heat from the gas being liquefied.

A further object of this invention is to provide a method of liquefyingnatural gas, wherein nitrogen is distilled from the natural gas andrefrigeration from the removed nitrogen is utilized to subcool arefrigerant used in the liquefaction of the natural gas. It is also anobject to utilize heat from the natural gas being liquefied for removalof a nitrogen from the natural gas, and then further cooling the naturalgas prior to expansion of the natural gas into the distillation zonewhere the nitrogen is removed.

A still further object of this invention is to provide a novel method ofliquefying natural gas by use of a plurality of refrigerants, whereinone of the refrigerants is methane, and boil-off vapor from the storagevessel receiving the liquefied gas is utilized in the methanerefrigeration cycle. Further, is is an object of the invention to returnexcess methane in the refrigeration cycle to the natural gas streambeing liquefied in such a mamner as to facilitate the removal ofnitrogen from the natural gas.

Other objects and advantages of the invention will be evident from thefollowing detailed description, when read in conjunction with theaccompanying drawings which illustrate this invention. 1

Inthe drawings:

Figure l is a partial flow diagram "illustrating a high temperaturelevelrefrigeration cycle which may be used .ina practice of this invention.

Figure 2 is a partial flow diagram of an intermediate temperature levelrefrigeration cycle which may be used in a practice of this invention,with Fig.2 being a continuation from the righthandend of Fig. 1. h

Figure 3 is a partial flow diagram illustrating a low temperaturelevelrefrigeration cycle which may be used 'ina practice of this invention,with Fig. 3 being a con 'tinuation from the right hand end .of Fig. 2,such that Figs. 1, 2 and 3 form a complete 'flow diagram illustrating apractice of this invention. V

The :process will hereinafter be described in detailwith reference tothe liquefaction of natural gas at asource of supply using an operativeset of temperature'andpressure conditions. It should be understood,however, that the conditions set forth are merely illustrative and mayeasily and properly be varied in .consonance with the design andcapacity of the apparatus, .the character of the gas from the standpointof composition, temperature and :pressure, and theconditions'under-which the liquefaction is carried out as influenced bythe volume of material, types of refrigerants and the like, all withinthe scope of the invention. In the example, the gas liquefied will be anatural gas from which moisture. acid gases. such as carbon dioxide,hydrogen sulfide and the like, will previously have been removed bypretreatment in the 'form of desiccators, amine extractors and the like.In this typical example, a cleaned natural gas is used having about 73mol. percent methane, about 12 mol. percent ethane, about 8 mol. percentpropane, about 2 mol percent nitrogen and minor percentages of heavierhydrocarbons. It will be understood that natural gas capable of beingprocessed in accordance with the teachings of this invention may have upto 20-25 molupercent heavier hydrocarbons, up to 20 mol. percentnitrogen, and up to 5 mol. percent carbon dioxide or hydrogen sulfide,but usually the amount of methane will be from '70'to more than 90 mol.percent of the natural gas.

Referring to the drawings in detail, and particularly 'Fig. 1, referencecharacter 4 designates a line leading from a source of supply of naturalgas (not shown) for conveying the gas to a liquefaction systempracticing the present invention. The natural gas. as previouslyindicated, will be passed from the producing wells through a clean-upsystem where moisture and acid gases are removed from the natural gasstream, and then the natural gas will be supplied to the liquefactionsystem. such that the natural gas will be supplied at a substantialpressure. For purposes of illustration, it will be assumed that thenatural gas conveyed through the line 4 will be at a presure of about715 p.s.i.a. and have a temperature of about 94 F.

The natural gas stream is passed in series through the tubes of threeheat exchangers 6, 8 and 10 by means of lines 12 and 14 for reducing thetemperature of the natural .gas to about 53 F. The three heat exchangers6, 8 and 19 are cooled by a relatively high temperature levelrefrigeration cycle, preferably utilizing propane as the refrigerant.Condensed propane is stored in a surge drum 16 at a pressure of about183.7 p.s.i.a. and a temperature of about 98 F. This propane is suppliedthrough a line 18 tothe shell of the heat exchanger 6 for initiallycooling the natural gas feed stream passed through the tubes of therespective heat exchanger. It may also be noted that a portion of thepropane refrigerant at about 98 F. is conveyed on through a line 20 foranother refrigeration operation, as will be described.

The propane being conveyed through the line 18 to the heat exchanger 6is expanded from about 183.7 to about 87.2 p.s.i.a. into the shell ofthe heat exchanger by means of a suitable expansion valve 22. The valve22 is controlled by a liquid level controller 24 mounted on one end ofthe heat exchanger to control the liquid level of the propane in theheat exchanger 6. The propane in the heat exchanger 6 cools the naturalgas feed stream from about 94 F. down to about 52 R, such that a portionof the liquid propane in the shell of the heat exchanger 6 will beboiled to provide both vapor and liquid propane in the heat exchanger 6.The propane vapor is withdrawn from the top of the heat exchanger 6through a line 26 and returned to an intermediate stage of thecompression portion of the refrigeration cycle, as will behereinafterdescribed. Liquid propane is withdrawn from the heatexchanger 6 through a line 28 and in turn fed to the shell of the mediumpressure propane heat exchanger .8. a

The propane conveyed through the line 28 will be at a pressure of about87.2 p.s.i.a. and a temperature of about 47 F. As this propane is beingfed into the heat exchanger '8 it is expanded down to a pressure ofabout 33.5 p.s.i.a. by use of an expansion valve 30, to substantiallyreduce the temperature of the propane entering in the heat exchanger 8,and remove an additional amount of heat from the natural gas feed streamflowing through the tubes of the heat exchanger 8. The natural gas willbe reduced from about 52 F. down to about 2 F. by passage through thetubes of the 'heat exchanger 8. The expansion valve 30 is controlled bya liquid level controller 32 mounted on one end of the heat exchanger 8.The transfer of heat from the natural gas to the propane in the heatexchanger 8 will, as before, provide a boiling of the propane in theshell, thus the heat exchan er 8 will contain both vaporous and liquidpropane. The propane vapor is withdrawn from the top of the shellthrough a line 34 and returned to another intermediate stage of thecompression portion of the refrigeration cycle, as will bedescribed. Theliquid propane in the heat exchanger 8 is withdrawn through a line 36and partially passed through an expansionvalve 38 .to the shell of'theheat exchanger 10.

The liquid propane flowing through the line 36 wi l be at about 33.5p.s.i.a. and have a temperature of about 6 F. The expansion valve 38 iscontrolled by a liquid level controller 40 on one end of the heatexchanger 10 to reduce the pressure of the propane being fed to the heatexchanger '10 down to about 10.8 p.s.i.a., such that the temperature ofthe propane is reduced to about -56 F. upon entering the heat exchanger10. Thus, the propane refrigerant in the heat exchanger 18 will furtherreduce the temperature of the natural gas stream flowing through thetubes of the heat exchanger, such that the natural gas discharged fromthe exchanger 10 through .the line 42'Wlll be at a temperature of about-53 F. As before, the transfer of. heat to the propane in the heatexchanger 10 will provide a boiling of the propane, such that propanevapors Will collect in the top of the shell of the exchanger 10. Thesepropane vapors are with drawn through a line 44 and returned to theinlet of the compression portion ofthe cycle, as will be described.

The major portion of the liquid propane in the line 36 is by-passed'around the heat exchanger 10 and fed on to a condenser 46 (Fig. 2) usedin condensing the intermediate temperature level refrigerant, as'will bemore fully hereinafter set forth. As shown in Fig. 2, the propaneentering the condenser 46 is expanded by a suit able valve48 down to apressure of about l1'p.s.i.a., with an accompanying drop in temperatureto about -55 P. which, it'will be noted, is slightly higher than thetemperature and pressure of the propane fed to the heat exchanger 10.Thus, the propane vapor produced in the condenser 46 may be withdrawnthrough a line 50 and returned to the shell of the heat exchanger 10 forjoinder with thevapor boiled off in the heat exchanger 18 through theline 44 to the compression portion of the cycle. The expansion valve 48is controlled by a suitable liquid level controller 52 on one end of thecondenser 46 to maintain the desired liquid level in the condenser 46.

As illustrated in the lower portion of Fig. 1, the compression portionof the propane refrigeration cycle utilizes three stages of compression,referred to as 54, 55 and 56, to progressively compress the propanevapors. The low pressure stage 54 receives propane vapors at about 9p.s.i.a., and at a temperature of about 2() F., from the line 44 andincreases the pressure of these vapors to about 32.5 p.s.i.a., with aresulting increase in temperature to about 59 F. These partiallycompressed propane vapors are fed to the intake of the intermediatestage 55, along with propane vapors from the medium pressure andintermediate temperature level exchanger 8 directed through line 34, aswell as propane vapors from lines 58 and 60. The propane flowing throughlines 58 and 60 is used in cooling the intermediate temperature levelrefrigerant, as will be described. It may be noted at this point thatthe pressure of the vapors withdrawn from the medium pressure heatexchanger 8 are at a pressure of about 33.5 p.s.i.a., slightly higherthan the discharge pressure of the low pressure compressor 54, such thatthe propane vapors generated in the heat exchanger 8 are returned to anintermediate stage of the compression portion of the cycle to minimizethe horsepower required for operating the compressor 54.

The intermediate stage compressor 55 increases the pressure of thepropane vapors to about 86 p.s.i.a., with a resulting temperature riseto about 80 F. These partially compressed vapors are fed to the intakeof the high pressure stage 56, along with vapors conveyed through theline 26 from the high pressure exchanger 6, and along with propaneconveyed through by-pass line 61 connected to the previously mentionedline 20 and propane vapors conveyed through the line 62 from theintermediate temperature level refrigeration cycle, as will bedescribed. It should be noted here, however, that the vapors produced inthe high pressure heat exchanger 6 are recirculated only through thehigh pressure compressor 56 to minimize the required horsepower for thecompressor stages 54 and 55.

The high pressure stage 56 increases the pressure of the propane vaporsto about 187 p.s.i.a., with a resulting temperature rise to about 122 F.The compressed vapors are conveyed through a line 64 to a suitableseparator 66 wherein any lubricating oil which may have been picked upby the propane vapors in their passage through the stages 54, 55 and 56is separated and collected in the bottom of the separator 66. It will benoted that at a temperature level of 122 F., lubricating oil present inthe propane vapor stream as a vapor mist and/ or fog will be condensedat least in part to permit separation from the propane vapors. Theseparated oil is selectively discharged from the lower end of theseparator 66 through a drain line 68. The cleaned propane vapors arewithdrawn from the top of the separator 66 through a line 70 andconveyed to a suitable heat exchanger 72 operating as a condenser forcondensing the propane. The condenser 72 may be easily cooled by waterat the temperature conditions existing, such that the propane passingthrough the condenser 72 will be reduced in temperature to about 98 F.and condensed. The condensed propane is fed through a line 74 to thepropane surge drum 16 for subsequent passage through the propanerefrigeration cycle.

The intermediate temperature level refrigeration cycle illustrated inFig. 2 may utilize either ethane or ethylene as the refrigerant,although ethane is the preferred refn'gerant, principally due to itsavailability is most natural gas. In other words, the ethane maynormally be obtained as a fraction from natural gas, the same as thepro-pane, such that a commercial system practicing the present inventionwill not require refrigerants supplied from an outside source. Thisintermediate temperature level refrigeration cycle utilizes three mainheat exchangers 76, 77 and 78 through which the natural gas feed streamis passed in series to incrementally reduce the temperature of thenatural gas from about 53 F. to about 145 F.

The stream is fed to the tubes of the heat exchanger 76 by the line 42and is withdrawn from the exchanger 78 through a line 80 at atemperature of about 77 F. The natural gas stream is in turn conveyed bythe line 843 to the heat exchanger 77 wherein the temperature of the gasis further reduced to about -107 F., and, in the example taken forillustration, at this temperature the natural gas will be substantiallytotally condensed. The feed stream is transferred from the heatexchanger 77 to the tubes of the heat exchanger 78 through a line 82;whereupon the temperature of the natural gas feed stream is reduced ondown to about 145 F. and the stream is withdrawn from the tubes of theheat exchanger 78 through a line 84 for further cooling, as will behereinafter described.

The intermediate temperature level refrigerant, which will behereinafter described as ethane, is stored in a surge drum 86 in liquidform at about 49 F. and at a pressure of about 96 p.s.i.a. The liquidethane is withdrawn from the lower end of the surge drum 86 through aline 88 and passed through a suitable heat exchanger 90 for a decreasein temperature to about 62 F., as will be described. The subcooledethane is conveyed on through the line 88 and expanded through asuitable expansion valve 21 into the shell of the heat exchanger 76. Theexpansion valve 91 reduces the pressure of the ethane to about 50.2p.s.i.a. which reduces the temperature of the ethane entering the heatexchanger 76 to about 80 F. for efiiciently removing heat from thenatural gas feed stream flowing through the tubes of the exchanger 76.The transfer of heat to the ethane in exchanger 76 provides a boiling ofthe ethane at the pres sure conditions existing to, in turn, provideboth vaporous and liquid ethane in the exchanger 76. The expansion valve91 is operated by a suitable liquid level controller 92 mounted on anend of the exchanger 76 to control the liquid level in the exchanger 76.The ethane vapor is withdrawn from the top of the exchanger 76 through aline 94 and returned through a heat exchanger 96 for refrigeratinghigher temperature ethane, as will be described. This expanded ethanevapor is conveyed on through the line 94 to an intermediate stage of thecompression portion of the ethane refrigeration cycle, as will be morefully hereinafter described.

Liquid ethane is withdrawn from the bottom of the heat exchanger 76through a line 98 at about -80 F., sub-cooled in a heat exchanger 100 toabout 90.5 F. and then fed to the shell of the medium pressure ethaneexchanger 77. An expansion valve 102 is interposed in the line 98adjacent the exchanger 77 and is controlled by a suitable liquid levelcontroller 104 for reducing the pressure of the ethane fed to theexchanger 77 down to a pressure of about 24.1 p.s.i.a., with a resultingdecrease in temperature of the ethane to about F. The ethane refrigerantin the exchanger 77 reduces the temperature of the natural gas feedstream to about -107 F., as previously indicated.

The transfer of heat from the natural gas feed stream to the ethane inthe exchanger 77 provides a boiling of the ethane refrigerant to in turnprovide both vaporous and liquid ethane in the shell of the exchanger77. The ethane vapor is withdrawn from the top of the exchanger 77through a line 106 at a pressure of about 23.5 p.s.i.a. and atemperature of about 85 F. This ethane vapor is passed through the heatexchanger 90 to provide a cooling of the liquid ethane being fed to theheat exchanger 76, as previously described, and is then fed on throughthe line 106 to the heat exchanger 96 to provide a refrigerating action,as will be described. The slightly warmed ethane vapor is then conveyedon through the line 106 to an intermediate stage of the compressionportionof the ethane refrigeration cycle, as will be described.

Liquid ethane is withdrawn from the shell of the heat exchanger 77through a line 108 at a temperature of about 1l0 F. and a pressure ofabout 23.5 p.s.i.a. for transfer to the shell of the low pressure ethaneheat exchanger 78 and a condenser 110 (Fig. 3) for a further cooling ofthe natural gas feed stream and for condensing the low temperature levelrefrigerant, respectively, as will be described. The ethane fed to theheat exchanger 78 (Fig. 2) is expanded by a suitable expansion valve 112down to a pressure of about 7.7 p.s.i.a., with a resulting decrease inthe temperature of the ethane to about l48 F. The expansion valve 112 iscontrolled by a liquid level controller 114 mounted on an end of theexchanger 78 to control the liquid level in the exchanger 78 in theusual manner. The ethane refrigerant in the exchanger 78 reduces thetemperature of the natural gas feed stream to about 145 F., aspreviously indicated, to provide a boiling of the ethane in theexchanger.

The liquid ethane fed to the condenser 110 (Fig. 3) is also reduced inpressure by an expansion valve 116 to a pressure of about 7.9 p.s.i.a.,with a resulting decrease in temperature to about 147 F. The expansionvalve 116 is controlled by a liquid level controller 118 mounted on thecondenser 116 in the usual fashion. Heat is transferred to the ethane inthe condenser 110, as will be described, to boil the ethane in thecondenser. The resulting ethane vapors generated in the condenser 110are withdrawn through a line 120 and directed into the shell of the lowpressure ethane heat exchanger 78 (Fig. 2); It will be noted that thepressure of the ethane vapors in the condenser 110 is slightly higherthan the pressure of the vapors in the heat exchanger 78, such that theethane vapors will readily flow from the condenser 110 through the line121) into the exchanger 78. All of the vapors in the exchanger 78 arewithdrawn through a line 122 and fed back through the heat exchanger 100for sub-cooling the liquid ethane being fed to the heat exchanger 77.These ethane vapors are conveyed on through the line 122 for passagethrough the heat exchangers 99 and 96, and then the ethane vapors arefed to the low pressure side of the compression portion of the ethanerefrigeration cycle. The pressure of the ethane vapors in the line 122will be reduced to about p.s.i.a. by passage through the exchangers 160,90 and 96.

As illustrated in the lower portion of Fig. 2, the ethane refrigerationcycle utilizes three compressor stages, referred to as stages 124, 125and 126, to progressively increase the pressure of the ethane vapors andprovide an economical refrigeration cycle. The low stage 124 compressesethane vapors from the line 122 to a pressure of about 22 p.s.i.a., witha resulting temperature rise of the vapors to about 121 F. This heat ofcompression is partially removed by a heat exchanger 128 preferablycooled by propane from the propane refrigeration cycle. As previouslyindicated, see Fig. 1, a portion of the propane liquid stored in thepropane surge drum 16 is conveyed through the line 26 to theintermediate temperature level refrigeration cycle. A portion of theliquid propane in the line 26 is expanded through an expansion valve 138into the heat exchanger 128 down to a pressure of about 87.2 p.s.i.a.,with a resulting temperature drop to about 47 F. The expanded propaneremoves heat of compression from the ethane vapors passing through thecoil of the heat exchanger 128 to such an extent that the temperature ofthe ethane vapors being fed to the inlet of the intermediate stage 125is about 60 F. After passing through the heat exchanger 128. the propaneis returned through the line 62 to the inlet of the high pressurepropane compressor 56 in the manner previously described.

The ethane vapors entering the intermedia e stage 125 are obtained fromthe low stage 124 and the line 106 leading from the medium p essure maine hane heat exchanger 77, .such that the ethane vapors withdrawn fromthe heat exchanger 77 are by-passed around the low stage 124 to minimizethe required horsepower of the compressor 124. The intermediate stage125 increases the pressure of the ethane vapors to about p.s.i.a., witha resulting rise in temperature to about 123 F. The heat of compressionin the ethane vapors is again removed by propane from the line 26expanded into a heat exchanger 134 in the same manner as previouslydescribed in connection with the heat exchanger 128. Thus, the ethanevapors flowing from the stage 125 to the stage 126 are reduced intemperature to about F.

The high pressure stage 126 also receives ethane vapors through the line94 leading from the high pressure main ethane heat exchanger 76, suchthat the vapors withdrawn from the exchanger 76 are by-passed around thestages 124 and 125. The high pressure stage 126 increases the pressureof the ethane vapors to about 107 p.s.i.a., with a resulting temperatureincrewe to about 118 F. Another heat exchanger 136 is interposed in theoutlet of the high pressure stage 126 and cooled by propane expandedfrom the line 28 in the same manner as in the exchangers 134 and 128.Thus, the ethane vapors discharging from the high pressure stage 126 arecooled down to about 60 F. These compressed ethane vapors are in turnpreferably passed through another heat exchanger 138 acting as anafterchiller to further reduce the temperature of the vapors to about 7F. The heat exchanger 138 receives propane through a line 140 leadingfrom the heat exchanger 136, with the propane fed to the exchanger 133being further expanded by a suitable expansion valve 141 to reduce thetemperature of the propane entering the exchanger 138 to about 6 F. Itshould be noted that a substantially greater amount of propane is fed tothe exchanger 136 than is fed to the exchangers 134 and 128, and thepropane fed to the exchanger 136 is reduced in pressure only to about87.2 p.s.i.a. Therefore, this propane may be withdrawn in liquid formfrom the exchanger 136- and in turn expanded into the exchanger 138 fora further reduction in temperature.

A portion of the propane is passed through the line 58 and expanded downtoabout 32.5 p.s.i.a. for entry into the intermediate stage propanecompressor 55, for the purpose of reducingthe suction temperature to thecompressor, as previously indicated. In the same manner, liquid propaneis flashed through line 61 for the purpose of lowering the temperatureof gas to compressor 56. The propane vapors discharging from the top ofthe exchanger 138 are returned through the line 60 to the intermediatestage propane compressor 55. It will thus be apparent that the propanerefrigerant is utilized to remove all of the heat of compression fromthe enthane refrigerant by use of heat exchangers between the variousstages of compression and a pair of heat exchangers after the last stageof compression.

The ethane vapors discharging from the exchanger 138 are conveyed by aline 142 to the heat exchanger 96 for a further reduction in temperatureof the vapors to about 40 F. As previously indicated, the exchanger 96is cooled by ethane vapors withdrawn from the main ethane exchangers 76,77 and 78. The cold methane vapors flowing from the exchanger 9-6 aredirected on through a line 143 to a suitable separator 144 wherein anylubricating oil which may have been picked up by the vapors in passagethrough the compressors 124, and 126 is removed. It will be apparentthat with the ethane vapors at a temperature of 40 F., any oil which maybe entrained in the vapors will be in liquid form and may be easilyseparated in the separator 144. These condensates are selectivelydrained from the separator 144 when and as required. The remainingethane vapors are directed from the separator 144 through a line 146 tothe coils of the'ethane condenser 46.

As previously described, the condenser 46 is main assess-r 9 tained at atemperature of about -'-55 F. by expansion of propane through the valve48, such that the ethane circulating through the coil of the condenserwill be converted to a liquid state. The condensed ethane flows from thecondenser 46 into the ethane surge drum 86 for re-use in the ethanerefrigeration cycle.

It will thus be apparent that the ethane refrigeration cycle utilizesthree progressive expansions of liquid ethane to provide three separatecooling steps for the natural gas feed stream, as well as three separatecompression stages corresponding in pressure levels to the expansionsteps. Thus, the ethane vapors produced in each of the first two coolingsteps may be returned to intermediate stages of the compression portionof the cycle to minimize the required horsepower for the compression ofthe ethane vapors. it will also be noted that the ethane refrigerant isused through a temperature and pressure range which permits the mostefficient heat transfer from the natural gas feed stream to the ethanerefrigerant and provide the maximum removal of heat from the natural gasfeed stream by the ethane refrigeration cycle. Further, the heat ofcompression is removed from the ethane refrigerant by the propanerefrigerant used in the higher temperature level removal of heat fromthe natural gas feed stream.

The low temperature level refrigeration cycle preferably utilizesmethane as a refrigerant and three main heat exchangers 148, 149 and150. The natural gas feed stream is fed to the high pressure methaneexchanger 148 by the line 84 to provide a further cooling of the naturalgas feed stream from about -145 F. to about l82 F. The natural gas feedstream is withdrawn from the exchanger 148 through a line 152 andconveyed to the re-boiler section 154 of a nitrogen stripping tower 156.The natural gas feed stream flowing through the re-boiler 154 provides awarming of the contents of the lower end portion of the tower 156, aswill be more fully hereinafter set forth, such that the temperature ofthe natural gas stream is reduced to about -204 F. upon discharge fromthe re-boiler 154. It may also be noted that a valved by-pass line 158is provided between the inlet and the outlet of the re-boiler 154 toby-pass all or a portion of the natural gas feed stream around there-boiler 154 when the conditions in the tower 156 so require.

The natural gas feed stream discharging from the reboiler 154 andthrough the by-pass 158 is conveyed by a line 16% to the coils of themedium pressure methane exchanger 149 for a further cooling of thenatural gas feed stream. The natural gas feed stream is cooled to about-2l6 F. by passage through the exchanger 149 and is withdrawn from theexchanger 149 through line 162. It will thus be noted that instead of anatural gas feed stream being conveyed directly from the high pressuremethane exchanger 148 to the low pressure methane exchanger 149, thefeed stream is first passed through the re-boiler section of thenitrogen stripping tower to obtain the benefit of the heat content ofthe natural gas feed stream in the operation of the stripping tower.

The natural gas feed stream is conveyed through the line 162 to a medialportion of the tower 156 for a distillation of nitrogen from the feedstream. The natural gas feed stream entering the tower 156 is expandedthrough a suitable expansion valve 164 down to an intermediate pressure,such as about 66 p.s.i.a., to enhance the vaporization of the nitrogencomponent of the feed stream. The tower 156 operates in the usual mannerto provide a downward flow of liquid and an upward flow of vapors. Areflux condenser 166 is provided in the upper section of the tower 156and is maintained at a temperature below the temperature of the expandednatural gas feed stream entering the tower 156, such that natural gasvapors rising through the tower 156 will tend to become condensed andflow downwardly through the tower, and

the vapors collecting in the upper end of the towerwill be enriched withnitrogen. The operation of the reflux condenser 166 will be describedbelow. Also, the nitrogen-enriched vapors collecting in the upper end ofthe tower 156 are withdrawn through a line 163 and utilized to cool themethane refrigerant, as will be described below.

The substantially nitrogen-free liquefied natural gas collecting in thelower end of the tower 156 is withdrawn through a line 17%) and conveyedto the coils of the low pressure methane exchanger 150. Methanerefrigerant in the exchanger further reduces the temperature of thenatural gas feed stream passing through the exchanger from a temperatureof about 209 F. to about 246 F. Thus, the low pressure methane exchanger150 acts as a sub-cooler. The liquefied natural gas feed stream iswithdrawn from the exchanger 15!) through a line 172 and expandedthrough an expansion valve 174 down to about atmospheric pressure, orslightly above, such as 17.7 p.s.i.a., into a suitable insulated storagevessel 176. The final expansion of the liquefied natural gas by thevalve 174 will, since the liquefied natural gas is sub-cooled, provide aminimum of flashing and minimize the vapors fed to the storage vessel176. It may also be noted that the expansion valve 174 is controlled bya suitable liquid level controller 178 on the side of the nitrogenstripping tower 156 to control the liquid level in the tower 156.

The storage vessel 176 is suitably insulated, and the liquid product,comprising substantially nitrogen-free liquefied natural gas at apressure of about 17.7 p.s.i.a. and a temperature of about 246 F., isselectively withdrawn from the bottom of the tank through a line 186,such that the liquid product may be either transported by pipeline, orloaded into suitable shipping containers (not shown) for transportationto distant markets. As it is well known in the art, the insulation ofthe storage vessel 176 will not be a perfect insulation. Therefore, atleast a minor amount of the liquefied natural gas will be boiled off asa vapor. This boil-off vapor is Withdrawn from the top of the vessel 176through a line 182 and fed into the methane refrigeration cycle, as willbe hereinafter set forth.

The methane refrigerant used for cooling the main heat exchangers 148,149 and 15% is stored in a suitable surge drurn 184 at a temperature ofabout l43 F. and a pressure of about 415 p.s.i.a., such that the storedmethane refrigerant will be in liquid form. However, at least a minorportion of the methane refrigerant in the surge drum 184 will be boiledoff, and this boil-off vapor is withdrawn through a line 186 to joinwith the nitrogen-enriched vapors withdrawn from the top of the nitrogenstripping tower 156 through the line 168. These vapors are used as afuel for various units of equipment required in a commercialinstallation practicing the present invention.

Liquid methane refrigerant is withdrawn from the bottom of the surgedrum 184 through a line 188 and fed to the high pressure methane heatexchanger 148. However, the refrigerant being fed to the exchanger 148is preferaby sub-cooled by an exchanger 190 down to a temperature of-l54.5 F. prior to being expanded by a valve 192 into the exchanger 148.The heat exchanger 190 is cooled by the nitrogen-enriched vaporswithdrawn from the top of the nitrogen stripping tower 156, as well asvaporized methane refrigerant, as will be described. The expansion valve192 is controlled by suitable liquid level controller 194 mounted on anend of the exchanger 148, such that the liquid methane refrigerant beingfed to the exchanger 148 will be expanded down to a pressure of aboutp.s.i.a., with a resulting decrease in ternperature to about -l85 F. Thetransfer of heat from the natural gas feed stream flowing through thecoils of the exchanger 148 will provide a boiling of the methanerefrigerant in the shell of the exchanger 148. The

resulting methane vapor refrigerant is withdrawn through a line 196 andpassed through a heat exchanger 198 to an intermediate stage of thecompression portion of the methane refrigeration cycle. The heatexchanger 198 is also cooled by the nitrogen-enriched vapors flowingthrough the line 168, as will as additional methane refrigerant vapors,as will be described, to cool compressed methane vapors leaving thecompressor portion of the refrigeration cycle.

Liquid methane refrigerant is withdrawn from the shell of the exchanger148 through a line 290 and passed through an exchanger 202 for areduction in temperature to about -l94.7 F. The exchanger 202 is cooledby methane refrigerant vapors, as will be described. T he liquid methanerefrigerant leaving the exchanger 202 through the line 200 is partiallyfed to the medium pressure methane exchanger 149 and partially to anintermediate portion of the nitrogen stripping tower 156 at a pointbelow the point at which the natural gas feed stream is expanded intothe tower 156. As will be apparent, the methane refrigerant will besubstantially nitrogen-free; therefore this refrigerant may be expandedinto a lower portion of the nitrogen stripping tower 156 and act as anitrogen purge to facilitate the removal of nitrogen from the naturalgas feed stream in the stripper 156. The expansion valve 204 used forexpanding the liquefied methane refrigerant into the tower 156 iscontrolled by a suitable liquid level controller 208 mounted on the sideof the methane refrigerant surge drum 184, such that only excess methanerefrigerant is fed to the tower 156 for joinder with the main naturalgas feed stre m.

The liquid methane refrigerant fed from the line 200 into the shell ofthe medium pressure exchanger 149 is expanded through a suitableexpansion valve 210 to reduce the pressure of the refrigerant enteringthe exchanger to about 66 p.s.i.a., with a resultant reduction intemperature to about 2l9 F. The expansion valve 210 is controlled by aliquid level controller 212 mounted on the end of the exchanger 149 inthe usual fashion. As before, heat removed from the natural gas feedstream flowing through the coils of the exchanger 149 provides a boilingof the methane refrigerant contained in the shell of the exchanger 149.The resulting methane refrigerant vapors are withdrawn from theexchanger 149 through a line 214 and passed through the heat exchangers190 and 198 to an intermediate stage of the compressor portion of therefrigeration cycle, as will be more fully hereinafter described.

Liquid methane refrigerant is withdrawn from the bottom of the exchanger149 through a line 216 and directed partially to the reflux condenser166 in the upper section of the tower 156, and partially to the lowpressure methane heat exchanger 150. That portion of the liquefiedmethane refrigerant directed to the reflux condenser 166 is expanded bya suitable expansion valve 218 down to a pressure of about 23.5p.s.i.a., with a resulting decrease in temperature to about -248 F. Itwill be observed that this temperature level is below the temperaturelevel (219 F.) of the natural gas feed stream expanded into the medialportion of the tower 156, such that any methane vapors rising throughthe tower will tend to be condensed and flow back downwardly to thelower end of the tower. It may also be noted that the expansion valve218 is controlled by a temperature controller 219 mounted on the vaporline 168 leading from the tower, such that the temperature maintained inthe reflux condenser 166 will be governed by the temperature of theoverhead vapors discharging from the tower. Methane refrigerant vaporsare withdrawn from the reflux condenser 166 through a line 220 andconveyed to the shell of the low pressure methane exchanger 150.

That portion of the liquefied methane refrigerant flowing through theline 216 which is fed to the low pressure exchanger is expanded througha suitable ex pansion valve 222 down to a pressure of about 21.5p.s.i.a., with a resulting decrease in temperature to about -250 F. forcooling the natural gas feed stream in the coils of the exchanger to-246 F. The expansion valve 222 is controlled by a liquid levelcontroller 224 mounted on one end of the exchanger 150.

Methane refrigerant vapor in the shell of the exchanger 150 is withdrawnthrough a line 226 and passed through the heat exchangers 202, 190 and198 to the low pressure side of the compressor portion of the methanerefrigeration cycle. These methane vapors will be provided both bycooling of the natural gas feed stream in the coils of the exchanger150, andby the vapor withdrawn from the reflux condenser 166.

The methane refrigerant is compressed by three stage compressors 228,229 and 230. The low pressure stage 228 receives methane refrigerantvapor from the line 226 which leads from the low pressure methane heatexchanger 150. It will also be noted that the boil-off vapor from thestorage vessel 176 is conducted through the line 182 and joined with thevapor in the line 226, such that this boil-off vapor is also fed to thelow pressure stage 228. The compressor 228 increases the pressure of themethane refrigerant vapor from about 15 p.s.i.a. to about 64 p.s.i.a.,with a resulting temperature rise to about 231 F. The temperature levelof the partially compressed methane vapor is sufliciently high that theheat of compression may be removed by a water cooled heat exchanger 232and the temperature of the vapor reduced to about 105 F. The majorportion of the vapor leaving the exchanger 232 is fed to the intake ofthe intermediate stage 229. However, a portion of the refrigerant may beby-passed through a line 234 and joined with the nitrogen-enrichedvapors in the line 168 to provide make-up fuel as necessary.

Methane vapors from the line 214 are joined with the partialy compressedvapors discharging from the exchanger 232 at the intake of theintermediate stage 229. It will be noted that the vapors in the line 214are taken from the medium pressure methane exchanger 149, such thatthese vapors are by-passed around the low pressure stage 228 to minimizethe horsepower requirements for the low stage of the compressor.

The intermediate stage 229 increases the pressure of the methane vaporsfrom about 60 p.s.i.a. to about 170 p.s.i.a., with a resultingtemperature rise to about 228 F. This temperature level is againsufficiently high that the heat of compression may be removed by anotherwater-cooled exchanger 236, such that the temperature of the vapors willagain be reduced to about 105 F.

The partially compressed methane refrigerant vapors discharging from theexchanger 236, along with vapor from the line 196, are fed to the intakeof the high pressure compressor 230. It will be noted that the vapors inthe line 196 are taken from the high pressure methane exchanger 148,such that these vapors by-pass the lower pressure stages 228 and 229 tominimize the horsepower requirements of these compressors. Thecompressor 230 increases the pressure of the methane vapors from aboutp.s.i.a. to about 430 p.s.i.a., with a resulting temperature increase toabout 2l0" F. This temperature level is again sufliciently high that theheat of compression may be removed by a Water-cooled exchanger 238 toreduce the temperature of the vapors to about 105 F.

It is also preferred to interpose an afterchiller 240 in the dischargefrom the exchanger 238 to further cool the compressed methane vaporsdown to about 60 F. The chiller 240 is cooled by propane refrigerant fedto the cooler from the line 20 through an expansion valve 242. Theexpansion valve 242 is operated by a liquid level controller 244 mountedon a side of the cooler to control the operation of the valve 242 andexpand the propane refrigerant from about 183.7 p.s.i.a. down to '13about 82.2 p.s.i.a., with a temperature drop to about 47' F. Propanerefrigerant is withdrawn from the cooler 240 through the line 62 andreturned with the propane refrigerant from the various other coolers inthe ethane compression cycle to an intermediate point in the compressingportion of the propane refrigeration cycle.

The cooled methane refrigerant vapors discharging from the afterchiller240 are conveyed through a line 246, through the exchanger 198, and onto a suitable separator 248. The compressed methane refrigerant vaporsare cooled to about 120 F. by passage through the exchanger 198, suchthat the methane will still be in vaporous form, but lubricating oilwhich may have been picked up as a mist, fog or vapor by the methane inpassing through the stages 228, 229 and 230 is condensed and removed inthe separator 248. The condensates are drained from the separator 248when and as required. The remaining methane refrigerant vapors are fedthrough a line 250 to the coil of the methane condenser 110, wherein asuflicient amount of heat is removed from the methane vapors by theethane refrigerant in the shell of the condenser 110 to convert themethane refrigerant to a liquefied state. The liquefied methanerefrigerant is drained into the methane surge drum 184 for a re-use inthe methane refrigeration cycle.

It will thus be observed that in the low temperature methanerefrigeration cycle, the methane is expanded through three separatestages for providing three separate cooling steps for the natural gasfeed stream. Also, the methane refrigerant is compressed in threeseparate stages, such that the expanded methane refrigerant may bereturned to intermediate portions of the compressor cycle to minimizethe horsepower requirements for compressing the refrigerant. The methanerefrigerant vapor is passed in heat exchange relation with the liquefiedmethane refrigerant prior to expansion of the refrigerant to obtain themaximum refrigeration from the methane.

From the foregoing it will be apparent that the present inventionprovides a novel method of liquefying a natural gas wherein the naturalgas is cooled in incremental steps by separate refrigerants, with therefrigerants being utilized through temperature and pressure rangeswhich provide the most efiicient transfer of heat from the natural gasto the respective refrigerants. The heat exchange between the variousrefrigerants and the natural gas feed stream is accomplished with therefrigerants being in liquid form to provide the most efficient heattransfer operation. It will also be apparent that the present inventionprovides a novel method of liquefying natural gas wherein nitrogen isdistilled from the feed stream by use of heat from the feed stream andby use of refrigeration made available in a refrigerant used forproviding a normal cooling of the feed stream. The natural gas feedstream is passed through the re-boiler section of a nitrogen strippingtower and then through a medium pressure methane exchanger to obtain thebenefits of heat from the feed stream, and yet provide operation of thevarious refrigerants at the maximum temperature levels to minimize thehorsepower requirements of the refrigeration cycles. Boil-off vapor fromthe storage vessel is fed to one of the refrigeration cycles, andexcessive refrigerant is returned to the natural gas feed stream in sucha manner as to facilitate the distillation of nitrogen from the feedstream.

Changes may be made in the combination and arrangement of steps andprocedures as heretofore set forth in the specification and shown in thedrawings, it being understood that changes may be made in the preciseembodiment disclosed without departing from the spirit and scope of theinvention as defined in the following claims. For example, and as shownin dashed lines in Fig. 3, a methane jet ejector 252 may be mounted onthe low pressure methane exchanger 150 and connected to the line 220leading from the reflux condenser 166. The ejector 252 is also connectedby a line 254 to the top of the separator 248. In this embodiment, thepressure available in the separator 248 may be used to operate theejector 252 and decrease the pressure of the methane refrigerant vaporin the line 220 and the reflux condenser 166. This will in turn reducethe temperature of the reflux condenser and reduce the natural gascontent in the overhead vapor line 168, with a correspondingproportional increase in the nitrogen content of the overhead vapors.Such a system is desirable when the overhead vapors from the tower 156are vented and not used as a fuel in the system.

While reference is made to three-stage compressors in the foregoingdescription, it will be understood that compressors having two or morestages may be employed and that each stage may be embodied in separateor multiple-stage compressor units. It will be further understood thatchanges may be made in the details of construction and operation withoutdeparting from the spirit of the invention, especially as defined in thefollowing claims.

We claim:

1. In a method of liquefying a natural gas composed mostly of methane,the steps of:

(a) supplying the natural gas in a process stream at an elevatedtemperature and pressure,

(b) condensing the natural gas without substantially reducing thepressure thereof by passing the process stream in heat exchange relationwith a series of separate progressively decreasing temperature levelrefrigerants for incremental reduction in temperature of the stream,

(0) expanding the liquefied natural gas into a storage vessel to apressure suitable for transportation of the product in liquid form,

(d) alternately compressing, condensing and expanding each refrigerantin a separate closed cycle wherein at least one of the refrigerants isexpanded to sub-atmospheric pressure, and

(e) passing the expanded refrigerants in heat exchange relation with theprocess stream at temperature and pressure levels such that the separaterefrigerants are in liquid form when passed in heat exchange relationwith the process stream and receive principally latent heat from theprocess stream to provide the condensation called for in step (b).

2. The method defined in claim 1 characterized further in that therefrigerants are fractions of natural gas.

3. The method defined in claim 1 characterized further in that at leastone of the refrigerants is methane, boil-off vapor from the storagevessel is added to the methane refrigeration cycle, and excess liquefiedmethane in the methane refrigeration cycle is fed into the processstream.

4. The method defined in claim 1 characterized further in that therefrigerants are propane, ethane and methane.

5. The method defined in claim 1 characterized further in that nitrogenis distilled from the process stream after the natural gas is condensed,but before the stream is expanded to a pressure suitable fortransportation.

6. The method defined in claim 1 characterized further in that a highertemperature level refrigerant is passed in heat exchange relation with alower temperature level refrigerant after compression of the lowertemperature level refrigerant for removing heat of compression from thelower temperature level refrigerant.

7. The method defined in claim 1 characterized further in that eachrefrigerant is compressed and expanded in a plurality of stages, andthat portion of each refrigerant vaporized at an intermediate pressureis returned to an intermediate stage of compression.

8. The method defined in claim 1 characterized further in that avaporized portion of each of the Iowertemperature level refrigerants ispassed in heat exchange relation with a condensed portion of the samerefrigerant for subcooling the condensed refrigerants.

9. The method defined in claim 1 characterized further in that threeseparate refrigerants are used at progressively lower temperature levelsfor cooling the process stream, each refrigerant is compressed andexpanded through a plurality of stages, and the higher temperature levelrefrigerant is passed in heat exchange relation with the intermediatetemperature level refrigerant between compression stages and after thefinal stage of compression of the intermediate temperature levelrefrigerant to remove heat of compression from the intermediatetemperature level refrigerant.

' 10. The method defined in claim 9 characterized further in that thehighest temperature level refrigerant is passed in heat exchangerelation with the lowest temperature level refrigerant to remove heatfrom the lowest temperature level refrigerant prior to condensationthereof. 11. The method defined in claim 1 characterized further in thatthree separate refrigerants are used at progressively lower temperaturelevels for cooling the process stream, and each of the highertemperature level refrigerants is passed in heat exchanger relation withthe next lower temperature level refrigerant following compression ofthe respective lower temperature level refrigerant for condensing therespective lower temperature level refrigerant.

12. In a method of liquefying a natural gas containing nitrogen, thesteps of:

(a) supplying the natural gas in a process stream at an elevatedtemperature and pressure,

condensing the natural gas without substantially reducing the pressurethereof by passing the process stream in heat exchange relation with aseries of progressively lower temperature level refrigerants forincremental reduction in temperature of the stream, with each seriescontaining more than one refrigeration stage,

(c) expanding the liquefied stream to an intermediate pressureinto themedial portion of a nitrogen stripping tower having a reboiler in thelower section thereof maintained at a temperature above the temperatureof the expanded stream and a reflux condenser in the upper sectionthereof maintained at a temperature below the temis 16 V a perature ofthe expandedstream for vaporizing and removing nitrogen from the stream,7

(d) withdrawing nitrogen enriched vapors from the top of the nitrogenstripping tower and passing said vapors in 'heat exchange relation withat least one of the refriger-ants, V

,(e) withdrawing the remaining process stream from the lower end of thenitrogen stripping tower,

(f) .subcooling said remaining process stream, and

. (g) expanding said remaining process stream into a storage vessel to apressure suitable for transporting the product in liquid form. 7

13. The method defined in claim 12 characterized further in that theprocess stream is passed through said re-hoiler prior to expansionthereof into the medial portion of the stripping tower for maintainingthe re-boiler at the desired temperature, and the lowest temperaturelevel refrigerant is passed through said reflux condenser formaintaining the reflux condenser at the desired tempe rature.

14. The method defined in claim 13 characterized further in that theprocess stream is passed in heat exchange relation with a refrigerantfor further cooling thereof after passage through the re-boiler andbefore expansion into the stripping tower.

, 15. The method defined in claim 12 characterized further in that thelowest temperature level refrigerant is methane, boil-off vapor from thestorage vessel is combined with the methane refrigerant, and excessmethane refrigerant is bled into the process stream.

16. The method defined in claim 15 characterized further in that theexcess methane refrigerant is expanded intothe stripping tower at apoint below the expansion of the process stream into the tower forcombining with the process stream and acting as a nitrogen purge in thestripping tower.

References Cited in the file of this patent UNITED STATES PATENTS2,495,549 Roberts Jan. 24, 1950 2,541,569 Born et al. Feb. 13, 19512,556,850 Gorzaly June 12, 1951 2,663,169 Twomey Dec. 22, 1953 2,696,088Twomey Dec. 7, 1954 2,823,523 Eakin et a1. Feb. 18, 1958

